Transposable elements are apparently parasitic DNA sequences that occupy a significant portion of the genome of most organisms. Using the Drosophila model system and theoretical population genetics analysis, the forces that control and shape the evolution of such elements are under study. By comparing the numbers of several retrovirus-like elements on the X and autosomes of individuals from a natural population it was possible to show that there is little role of natural selection against a mutant phenotype of the insertions themselves. Other experiments showed that inversions, which suppress crossing over, allow the accumulation of transposable elements. Theoretical models of the possible role of asymmetric crossing over between members of the same family of elements in the elimination of elements and thus the containment of copy number in the population were analyzed. Ongoing experiments are directed toward the relationship of the asymmetric crossing over to normal crossing over and the distribution of elements along the various chromosomes.